The invention relates to an intervertebral implant with a top contacting element, a bottom contacting element and arranged between these a core which with an upper, spherical bearing surface engages a spherical bearing socket, having the same radius, of the top contacting element and with a lower, spherical bearing surface engages a spherical bearing socket, having the same radius, of the bottom contacting element.
Such an intervertebral implant is known, for example, from U.S. Pat. No. 5,401,269. Herein the bearing surfaces and the associated bearing sockets on the upper side and on the underside of the core have the same radius, i.e., the core is lens-shaped. Pivoting movement of the two contacting elements relative to each other automatically results in a translational movement of the two contacting elements towards each other. Moreover, no defined kinematics are achievable with such an arrangement. For, upon pivoting movement of the contacting elements relative to each other, there are various possibilities of relative movement, depending on whether upon pivoting movement of the two contacting elements relative to each other, the core remains unpivoted in one contacting element and upon pivoting movement only the other contacting element moves relative to the core, whether conversely the core pivots only relative to the one contacting element and remains unpivoted relative to the other contacting element or whether a situation between these arises where both contacting elements are displaced relative to the surface of the core. If the core remains at rest relative to one of the two contacting surfaces during this pivoting movement, the entire movement of the contacting element takes place exclusively around the center point of the bearing surface relative to which the contacting element pivots, i.e., in one case around the center point of the top contacting surface, in the other case around the center point of the bottom contacting surface, and if the core moves relative to both bearing surfaces, an undefined movement results, which is made up of pivoting movements around both center points.
In particular, in order to keep the undesired translational movement as low as possible, it is also known to allow the core to engage on one side only with a spherical bearing surface in a corresponding bearing socket of a contacting element and to provide on the opposing side a plane bearing surface between the core and the other contacting element (U.S. Pat. No. 5,507,816). The core can then deviate laterally on the plane bearing surface and compensate the undesired translation. Owing to the relatively high forces acting by way of the ligaments and muscles on the adjacent vertebral bodies, the vertebral bodies are clamped strongly against one another and, upon pivoting movement of the contacting elements, this may result in the core being driven laterally out of the space between the contacting elements, i.e. the core will possibly move on the plane bearing surface further out of the position of rest than is expedient for the physiological bearing of the vertebral bodies.
The object of the invention is to so design an intervertebral implant of the kind described at the outset that, on the one hand, a defined movement sequence can be ensured and that, on the other hand, the undesired translational movement during the pivoting of the contacting elements is kept as small as possible.